AA filter question - Joe?

Started Feb 9, 2012 | Discussions thread
GordonBGood Veteran Member • Posts: 6,306
Re: Reducing moiré...

Peter 13 wrote:

GordonBGood wrote:

Barry Margolius wrote:

If you shoot with a camera without an AA filter and get a shot that shows significant moire, can it be effectively removed in post processing? Would it be easy? Hard? Impossible?

Since I've never had a camera without an AA filter, I'm curious.

Barry, there are techniques to reduce or remove moiré in post processing, as follows:

1) The AmaZe demosiacing algorithm (available in the free RawTherapee - http://www.rawtherapee.com ) can greatly reduce to almost eliminate moiré at the cost of extra processing time, but also has a tendency to produce other image artefacts.

2) One can use a image editor with editors to apply the pattern of the green (generally much less moiré) luminance to a blurred version of the colour channels for a masked area that has moiré to greatly reduce or eliminate the moiré pattern.

Note that both of the above have the limit that there has to be a signal (ie. the green channel which is normally sampled at a higher rate for Colour Filter Array (CFA) Bayer pattern images), so if there is a very weak to no green component in the colour exhibiting the moiré, the correction becomes less and less possible. Single red or blue colour pattern typically cannot be fixed, but fortunately pure reds or blues don't occur much naturally.

Moire cannot be masked but the best way is not to allow it in the first place.

Moiré can't be masked for the channels in which it appears ; however, the above techniques work because for a Bayer pattern sensor the green channel is sampled at a higher rate than the colour channels and therefore tends to have less or little moiré patterns in the first place. Thus, using the green channel as the luminance signal and restoring the colour information from a very blurred version of the colour channels, one ends up with an image that appears to have less moiré yet retains the detail. As specified, this works best when only applied to the problem moiré areas and under the conditions mentioned.

GordonBGood

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